Thin film resistors are commonly used as passive devices in mixed-mode integrated circuit devices. Traditionally, thin film resistors are fabricated using polysilicon. The resistivity of the polysilicon is manipulated by choosing appropriate polysilicon deposition temperature, pressure and doping concentration. Changing the doping concentration of the polysilicon layer allows the fabrication of resistors with desired resistance. However, it is difficult to maintain the stability of a polysilicon resistor because it is subject to grain structure evolution (from the different thermal cycles of the fabrication process) and hydrogen-concentrations emitted from surrounding dielectric materials (e.g., Plasma-Enhanced CVD Silicon-Nitride). Although polysilicon is routinely used in manufacturing, the controllable resistivity range is typically ˜10−3–10−2 Ω·cm (or Sheet-Resistance of approximately 100–1000 Ω/square for 1000 Angstroms film). The sheet resistance can be changed by increasing or decreasing the film thickness. However, the uniformity of polysilicon thin film degrades as film thickness decreases. Also, grain structure dependence on thickness and limitations of printing and patterning capability limit the increased resistance that can be obtained by varying thickness. Also, it is difficult to control the quality of the product due to the diffusion of doped materials during subsequent processing steps when a wider range of resistances are needed.
Typical metal-based thin film resistors have low resistivity (in the range of 10−5 to 10−3 Ω·cm). For high resistance applications, Chromium (Cr) based thin films have been developed. Chromium films have a relatively high resistance 0.1–1 Ω·cm. Though Chromium films provide a relatively high resistivity, there is a need for thin-film resistors having even higher resistivity (higher than ˜1 Ω·cm). Also, chromium is not commonly used in standard VLSI processing techniques. Thus, methods that use Chromium-based thin films will be difficult and costly to implement into conventional manufacturing processes.
Titanium nitride that is deposited using chemical vapor deposition (CVD TiN) is commonly used as a barrier material in semiconductor fabrication processes. However, CVD TiN film is not commonly used in thin film resistor applications as it has poor resistance stability. Also, the resistivity of a conventional CVD TiN film (typically 10−2 to 5×10−2 Ω·cm) is not high enough for most applications. Moreover, the CVD TiN films' resistance drifts with time when the film is exposed to ambient. This instability imposes a serious limitation to the film's usage as a resistor. As the footprint of advanced IC devices has decreased, there is no longer sufficient “real estate” for the layout of long wired resistors. Accordingly, there is a need for a resistor that takes up less “real estate” on the semiconductor device.
Accordingly, what is needed is a thin film resistor and a method for forming a thin film resistor having good resistance stability with different process flows (i.e., thermal cycles, ambient, etc.) and having a wide range of resistivity. Also, a thin film resistor and a method for forming a thin film resistor is needed that meets the above needs, that uses less semiconductor “real estate” and that can be easily implemented into conventional manufacturing processes. The method and apparatus of the present invention meets the above needs.